1. Field of the Invention
The present invention relates to a drive assembly for a track-type vehicle.
2. Background Art
One drive assembly used in the prior art for a track-type vehicle is provided with bogie (or rocking type) track rollers for improving the riding comfortableness at the running time of the vehicle (as referred to FIG. 1 to FIG. 5 of U.S. Pat. No. 6,607,256). FIG. 7 is a side elevation of a bulldozer as the track-type vehicle disclosed in U.S. Pat. No. 6,607,256, and FIG. 8 is a side elevation of a drive assembly of the track-type vehicle. In FIGS. 7 and 8, the track-type vehicle is provided with a drive assembly having crawlers 6 below the right-hand and left-hand sides of its body. The drive assembly is provided with a track frame 1 arranged along the longitudinal direction of the vehicle, and an idler 2 and a sprocket 3 disposed on the two longitudinal end portions. Below the track frame 1 and at the position closest to the idler 2, the drive assembly is provided with a track roller 43, which is mounted rotatably at the distal end portion of an arm 41 of a single track roller unit 40. The arm 41 is so attached turnably to the track frame 1 as can be vertically rocked on a pin 42. Below the track frame 1 and between the single track roller unit 40 and the sprocket 3, on the other hand, there are provided a pair of longitudinal track rollers 11 and 12, which are mounted rotatably at the two end portions of a second arm 32 of a double track roller unit 10 having a first arm 31 in addition to the second arm 32. The first arm 31 is so attached at its proximal end portion to the track frame 1 as can be vertically rocked on a pin 33, and the second arm 32 is so attached at its central portion to the distal end portion of the first arm 31 as can be vertically rocked on a pin 35.
On the abutment portion between the distal end portion of the arm 41 of the single track roller unit 40 and the lower portion of the track frame 1 and at the abutment portion between the upper portion of the distal end portion of the first arm 31 of the double track roller unit 10 and the lower portion of the track frame 1, respectively, there are mounted elastic members 44a and 44b, and 34a and 34b, which have predetermined moduli of elasticity. The loads to be applied to the track roller 43 of the single track roller unit 40 and to the track rollers 11 and 12 of the double track roller unit 10 are respectively borne by the upper and lower elastic members 44a and 44b, and 34a and 34b through the arm 41 and the arms 32 and 31.
Here in the drive assembly for the track-type vehicle according to the prior art described above, the position for the track roller to support the vehicle load is located at the elastic members 44a and 44b in the single track roller unit 40 and is substantially identical to the position of the track roller 43. In the double track roller unit 10, on the contrary, the support position is located at that of the elastic members 34a and 34b and is substantially identical to the center position between the paired front and rear track rollers 11 and 12. Usually in the track-type drive assembly, the distance between the track rollers on the longitudinally outermost sides for supporting the vehicle load is called the “effective ground contact length”, by which the stability at the vehicle running time is retained. Therefore, the effective ground contact length of the running device of the prior art is the distance between the track roller 43 of the single track roller unit 40, as located closest to the idler 2, and the center position between the front and rear track rollers 11 and 12 of the double track roller unit 10 located closest to the sprocket 3, so that the length is substantially shorter than the distance between the outermost track rollers. Merely in order to enlarge the effective ground contact length, generally speaking, it is naturally sufficient to elongate the distance (which will be called the “tumbler distance”, as usually called so) between the idler and the sprocket thereby to elongate the distance between the outermost track rollers accordingly. With this modification, however, the track frame 1 is large-sized, and the running length of the crawler 6 is enlarged to raise the cost. Therefore, it has been desired to make the best use of the space within the tumbler distance thereby to make the effective ground contact length as large as possible without elongating the tumbler distance.